What generally happens is that the actor is placed in front of a camera and is instructed to perform selected lines and actions to see if they are suitable. The developed film is later played back for the relevant production personnel for evaluation.
Screen tests can also be used to judge the suitability of costume, make-up and other details.
Different types of actors could have different tasks for the tests. For example, a lead for a musical could be a request to sing and dance to demonstrate their skill. International actors like Bruce Lee were given screen tests to demonstrate that they are sufficiently articulate in the relevant language. In Lee's case, he was instructed to converse about Chinese culture in English to judge his grasp of the language and then to demonstrate some martial art moves to show off his physical skills.