Remember to own the space in the room, and to own the time. It's all yours. One of the most common errors actors make is to rush themselves. They'll wait for you. That's not lip service.
Filmmakers take actors very seriously.
Films cannot be made without actors, and if you take the moment to assert yourself professionally, the respect you have for your own work will earn you their respect. Once you are settled and feel ready, begin. Take a moment to let the reality you created take over. Don't just jump into your lines. If you do not have the first line, go ahead and let your prep take over before you say that you are ready.