Well in the first half is about very different things, and not protecting a woman. She has no reason to be in plot till the second act. The love interest is has nothing to do with the villains, and there is nothing she can do to help the hero. The villains don't even know her and have no reason to go after her. Even though the is a love interest, she is not as main of a character and only serves the last half of the plot when she has a reason to risk her life and help.
Act One is where you introduce main characters important to the story. Star Wars starts with Leia and then she appears only fleetingly as a hologram message until her rescue in Act 2. By putting in an appearance, the audience is clued in that "this person is important later, watch for them." It doesn't matter if the protagonist and antagonist don't know her at this point. Even if she is only given a brief close up witnessing the crime, you have made her a key element. At a slightly later point you show her trying to contact someone about it then change her mind. Obviously she can't call the police. She is unsure how to help. After two appearances, the audience is clued in. So when she reappears in Act 2, they know she is a critical feature.
I could introduce her in a scene in the first act though, between pages 15-30 about, if she needs it. The scene will have nothing to do with the plot at hand though, and it will just be to introduce her. She can have a lunch scene with the hero or something like that, if it's not risky to break away from the plot for it.
If she's a love interest, you also need to invoke a bit more contact. Yes, they can have casual sex. But love interest is usually a bit more involved. She may not know how he can help her or vice versa. In fact when she learns more about him on the date, she may become frightened which is why we don't see her again until Act 2.
"Gee, you seem like a swell guy. What do you do?"
He smiles, "I'm a cop." She freezes.
So you say I should flip the two around. The antagonists commits some terrorists acts in the first two scenes, that get things going. So I should move those to page 11-20? However every guide I read says to get to the story quickly. Wouldn't it be more quick to start out with the first terror attacks and the theme, then go from there with protagonist's investigation which would be the challenge? I can introduce the characters from pages 1-10 beforehand, but I got nothing for them to do that is plot related, before the attacks. I was also told by few other writers, that when it comes to action thrillers, I should jump into the action in the first scene, if that's true. But if the Inciting Incident does not cause the opening action scene, what should?
I've never read your script. GENERALLY you want to start strong to grab attention THEN you go back and lay some groundwork. Think of most of your crime shows. They start with a crime, then flip back to "it's an ordinary day at the station". We get to see some of the key officers react when the call comes in. Now they jump into the preliminary aspects of the investigation. Then they find the one thing--dead kid, dead puppy, etc.--that creates the resolve to solve the crime. Then Act 2.
So start with your terrorist act. But realize it won't immediately impact all of your characters. Your love interest may be enjoying a dinner with the hero when he gets a call to investigate. He apologizes and leaves. Focus on her reaction learning that there's been a bombing. That's the last we see of her until Act 2. She may talk to the hero by phone once or twice putting off meeting with him for various reasons until Act 2.
If it's just an explosion, it may be gas line leak. It will call in the front line police and firefighters. It won't necessarily be viewed as a terrorist act immediately. And you might not want to even clue the audience in immediately that it's a a terrorist act. It's a big boom. Then switch to the police station and have the terrorist threat come in promising more. Now confusion is amplified to chaos in the wake of the explosion.
Dramatic tension is like a rollercoaster. "Classic" rollercoasters climb you up the hill before releasing you. Modern rollercoasters like to start with a series of building adrenaline pumpers. Starting with action gets the audience into the story but then you need to back off and tell them who the key players are so they can make sense of who to follow.
I was also originally planning on having the antagonist disappear after his intro, and does not return until The Big Whammy. When he returns, you also get the Why of what he is doing. He has nothing to do but sit behind a desk, and hide and his thugs do his work for him, up until that point, if that's okay. But the antagonist is not really the main one. There is a main antagonist, for the first half, but it's a twist, and we're lead to believe he is on the protagonist's side the whole time. Is that okay?
That's fine. Except you want the audience to believe that the guy behind the desk is your supervillain so that the twist hits them hard. To do that, your desk villain needs to have a bit more screen time. You can do this by having him on the phone speaking very generally.
"What the hell happened? You weren't supposed to kill anyone!"
Throwing in occasional lines like that make the audience wonder about his involvement. They may even be sympathetic until the reveal. After the reveal they think okay we know what's happening. Then you last exposure of the 'real villain' is all the more powerful.
There is also this article on the 3 act structure, if any of it is true, I'm not making judgments.
http://www.truby.com/threeact.html
People need to understand that Structure is different from Story. The structure does not tell you how to write your story. Nor does every story have the same structure. The storyline of the Hero's Journey and its structure have been used as a template that has become adopted--indeed canonized--as the basis for most movies. As a result, most movie analysts try to cookie cutter all movies into the story structure of the Hero's Journey and make every plot element fit the Hero's Journey storyline.
For beginning screenwriters, it is a good template to follow. They see it all around them. They learned it as kids-"Once upon a time, there was a young fellow named Jack. And he ran into a frog. The frog said 'would you like to go on a trip with me?' Jack said, 'If it isn't far. I have to be back by lunch.' And the two set out. (Act 1)" and so forth. For many writers, it is the only tool in their toolbox. But with practice and research you realize you need more than a hammer to build a house. So you get screwdrivers, saws, a few powertools and look at different blueprints, learn about plumbing and wiring. Eventually you build a house. Most of us build treehouses first with scrap lumber, a standard blueprint, a hammer and some nails.
If you're starting out
master the Hero's Journey template. When it's second nature, then you begin experimenting.
The movie industry likes selling houses with amenities, not treehouses. It's not the structure that's flawed but the perceptions. I guarantee that 99.9% of all houses have the floor (Act 1), the walls (Act 2) and the roof (Act 3). Some have basements, staircases, sunrooms, etc. The best have sinks, toilets, central air, power, lights, etc. As well as sweet locations.