Study script / screenplay
Be sure to go over the script or screenplay of the short film that you are looking to find actors for. Once you understand the core concepts you will be able to develop a character profile for the potential actor you want to bring in to your project.
Place ad in classified section
Place an ad in your local newspaper classified section stating your project and the need to hire actors for the short film. You can also put an ad in movie, television and theatre trade magazines for a more targeted approach.
Go online and place an ad on different websites that are frequented by actors looking for work. Be sure to place your ad on reputable websites for adding credibility to your short film. You will also find many different actors having their own personal websites which you could look over in detail.
Hire an agent
Find a decent agent and ask them to find someone that fits your criteria for an actor in your short film. Be sure to give them all of the details of your project so that you can get a good response. Please note that using an agent will cost considerably more than just putting ads out looking for actors.
Place flyers at local theatres
If your budget is limited then you can look for actors at theatres that put on stage plays and other types of entertainment. Usually these stage actors are looking to make extra money and do not mind being a part of small amateur projects.
Visit local university acting department
You can always visit your local university acting department to find some actors that might be willing to perform in your short film for a relatively low cost. Be sure to give all the details of your short film to as many people as possible to spread the word of your project.