There will be plenty of time to show your character in action in Act I, which is the real world, the thesis - we use this act to show them in action what they do (is she a doctor?), how they do it (is she the best and what's her style, mannerisms, attitudes, etc.?). So one can add some of that to your character's intro but if the idea is to keep the writing process slim and concise, why add all that stuff there when you have other real estate in the act to do so?
Character entrances are one of the places the writer gets to show their style, flex their creative muscle if you will. This is where one should use a powerful metaphor no longer than one sentence to introduce/reveal a major character. Like the description of Teddy Larson in the movie Body Heat when simply introduced as "Teddy Larson, rock-and-roll arsonist". Or NEO (The Matrix) described as "a young man who knows more about living inside of a computer than living outside of one". I tried to emulate that when I introduced my protagonist in my short as..."a chest match cloaked in a game of checkers" (yeah, yeah I know it's not the best but that's besides the point). But it allows the writer to do their thing with words (hey it's the allure of the writer). Plus it helps us get the reader to do what we want from them - turn the page. When done right, this technique can force a reader to go digging for more about this character, thus turning the page. Plus when the pros do it, it helps sell a story to an actor. Actor's like complex characters, interesting characters and the powerful use of words can help paint that picture in your story as early as possible instead of forcing the talent to have to read further into the act (hook them early and keep them hooked). This style also gives the talent some room to imagine and bring their own interpretations to the character.
One more thing, descriptions of clothing and/or character specifics (race, age, size) should be used sparingly (and by that I mean included only if necessary). While these things help casting and wardrobe they can also be restrictive. Remember, if you are selling your script leave such decisions open as you never know who the buyer will have in mind or can get to play the part. Your job as the writer is to tell a story so do just that. Besides there are ways to craft such suggestions. Why go into the detail of a dressing style when you can just say your character is Goth, or a teenager, or a tight-ass? Clothing styles come and go but character traits tend to be fixed.
Hope that helps.
