When casting a film, there are two schools of thought. The first is to attach actors and then go looking for the financing. The second is to secure the financing and then go out to talent. Either way works.
If you are a known producer or director you will have an easier time attracting talent prior to having your financing in place then if you are a first timer (unless Brad Pitt is your best friend). Even then, most actors will not completely commit to a project unless the funding is in place. This is for a lot of reasons.
The first is that actors have agents and managers looking out for their interests. These representatives may allow their clients to lend their names to projects if the rest of the package is solid or if there is a personal connection to the material - like the actor's production company owns the rights to the script - but short of that, there is no real incentive for them to be "locked" into doing the project if the financing is not in place.
The second is that they realistically cannot say whether they will be available or willing to do the film for an amount affordable to the producer as and when the picture is funded.
Lets say that in a purely fictitious example you had a deal with Kiefer Sutherland before he signed on to "24". He was a popular and well paid actor before the show, but let us hypothesize he wasn't getting the roles he wanted or the money he deserves. He commits to your project and you agree to pay him his usual fee and off you go to find the money.
Then, he gets the role on "24" and a month later you say you have the money to go into production immediately. He is no longer available. Your investors agree to wait until after the first season wraps. But Kiefer wants to spend time with family or he gets an offer of double that amount to do a film right away. If he is locked in, then he misses out on better opportunities. If he is not locked in, this scenario can continue for years until either the project goes away or he is available and you have enough money to afford his new quote.
A riskier version of this is that you agree to make a pay-or-play offer. This essentially means that if you do not start production on a certain date you owe the money, or a portion of it, to the actor regardless. This may be a good route if you are confident the funding will materialize in time, but otherwise just a gamble.
I prefer to have my funding in place before going out to talent for many reasons. What I have written above is part of it. The other is that you never know who you are going to get interested in the project.
Lets say that I have the financing in place with $200,000 available for the main star. I hire a casting director and the project goes out to the breakdowns. I not only get the benefit of having my casting director pitch the project to the agencies, but the agencies can pitch their clients to the casting director.
Suppose I get really lucky and a manager knows his client is dying to do a film based on the subject of the project. The actor is available and will do it for the amount I have despite the fact that they normally get double that amount. Win Win.
The other part of that equation is the word "available". Actors that are in demand are usually working or getting ready to work. If you are ready to go into production in a week and the actor is booked for three weeks you may decide to push your production start date for that actor. At least you know where you stand from the get go.
Additionally, actors are human and have human needs like all of us. If an actor had a need for additional cash to cover an unforeseen financial obligation, they might want to get their next job sooner than later. Or, perhaps they have a child going to college and another baby on the way and need to plan for that with an added chunk of cash. Sometimes, you can get better talent than you can normally attract for these human reasons. I wouldn't count on it, but it is human. And certainly this happens less, if ever, with the big stars.
When casting, I like to know who is hot directly from the marketplace. Internationally it has always surprised me to learn who is popular in Germany or Japan for example. The US market is somewhere between 40% to 60% of the market depending on what day it is. That means that foreign territories are worth about the same. If you know which male actors are popular in a territory that is worth 10% of the overall market, that is an actor you should be considering.
Needless to say we are not talking about the big box office actors who work everywhere. This is, after all, a blog about indie filmmaking. We are talking about the actors that will do a potentially non-theatrical film. The list of these names change day to day.
Finally, if you can afford it, I honestly suggest you hire a casting director. Not only can they save you time in your search by giving you realistic expectations, but they also know who will be available, when. They also have access to those Hollywood agents that might not return your calls.
Most important of all, casting directors know how to negotiate the finer points of a SAG contract and how to deal with the requests of talent or of agents/managers on behalf of their clients. Ultimately it is up to you, the producer, but they can certainly give good advice.