It depends on the person. It can be simply their interest and belief in the project, that they like and trust you, the sign in bonus amount (if any), fear of you offering the role to someone else, the fact that you won't pitch the project without the agreement etc.
The agreement is similar to a writers script optioning agreement. Found this one in Google. I give you no guarantees of quality, so I suggest you take it to your lawyer.
http://sonnyboo.com/downloads/individ/optionagreement.pdf
or another one:
http://www.absolutewrite.com/screenwriting/screenplay_option.htm
You'll have to change it for actors... or use an actors agreement I also found in google:
http://www.docstoc.com/docs/65170245/Actor-Option-Agreement
or another one:
http://270legal.com/actor-agreement/
It's amazing what you find in google, right?
You'll want to consider one from the area that you're going to be shooting. Budgets are quite geographically sensitive. Without this, it's a guesstimate, which really is what a line producer does anyway before the UPM/First AD comes on and locks in the schedule and budget.
Producing is tough. Lots to learn here too. Just note, I'm not a producer, just a producer-in-learning / producer-wanna-be so take my advice with a grain of salt.