Everyone tries to attach name actors to their project as they raise money with the expectation that the presence of known stars will make the funding easier. I believe that expectation is valid. Most investors would be more inclined to part with their life savings for a project with a marketable actor attached. Foreign distribution companies still pay money up front for projects with noted talent connected. Having a name attached to a project is a huge step in seeing your film made.
Here's a few thoughts on this process:
1) Everyone wants to attach talent, but agents WILL NOT look at scripts that aren't funded. Why would they? If Joe Hollywood receives five offers a day with real money attached, why would he bother reading your script with only the possibility of money attached? Why would an agent, who's goal is to make money and feed their family, spend time with a script that doesn't have a guarantee of actual income? Even when you make a real offer with a funded project the agency will normally string you along for as long as they can just in case something better comes along. This is why it's generally better to make an offer as close to production as possible. Make it a few months out, and you might lose the talent you want anyway.
2) What is an Offer? An Offer is a job with real money and real dates. Be careful when you approach an agent hoping to attach Joe Hollywood to a project that you don't actually make an offer. If you suggest an actual payment value, you are crossing the line into "Offerdom." We'd like Joe Hollywood to star in our film, we'll pay him $100,000 can be interpreted as an Offer. Once an Offer is made, and accepted, you are obligated to pay the performer, whether you make the movie or not. An Offer locks you into a Pay or Play agreement. So if you make an offer, then never raise the funds, the agent can demand payment. I don't think many agents would if your intentions are good, but it's not worth the risk. Most agents will ask, Is this an Offer? This means, do you have money to back it up and are you willing to enter into this agreement? I have a friend who Offered a lead role to an actor, then the opportunity to hire a bigger name came along. They fired the first actor to hire the bigger one. The agency insisted on payment. They ended up casting the first actor in a smaller role, but still had to pay him the total offered for the lead role.
3) How do you attach talent? Is this your first feature? Are you shooting for under a million? Are you a regular indie film person with no industry contacts? If you've answered yes to any of these, you DON'T. Write a movie that has great parts for talented actors. Raise your money, then approach appropriate talent. Actors love to work, so if you have a project and can pay, you will have a lot of people to choose from for your film... but until you have the money, you're not legit.
4) The best way to attach talent is to be friends with talent, or people who know talent. To get in through the back door, so to speak. But it's unlikely anyone of consequence is going to want to risk working with you until you've proven yourself. So make a no-budget feature and get into some film festivals, so you have some laurels to lean on. As a producer, it became so much easier to talk to agents once I could say, "I produced a movie called White on Rice," which agents know simply because it had a theatrical release. Also, having connection to an agency is helpful. If you can be signed by an agent yourself, you have a pool of talent resources within that agency that you can go to.
5) On some projects I see people attach below the line (non-talent) people, like cinematographers, etc. This doesn't make sense unless it's some kind of legend or Oscar winner (or simply part of your usual team). Some people feel that "crewing up" at the development stage makes you look more like a real movie, I think it makes you look amateurish. It's not hard to call a composer, cinematographer, editor and ask if they'll work for you if you raise the money. Film professionals work for a living. They'll say yes to any job that promises to pay. Focus on raising the money, then work out the details.