I covered this topic a little previously, now I want to spend an entire post on it.
When you're on a project, just because you're done with your tasks does not mean you're finished working on the project. Whether it be coding, art, producing, management, whatever. You're not finished until you move onto another project. If you chill and celebrate early, your other teammates (who continue to work) will wonder why, and begin to resent you. They don't want to spend late nights and weekends working to finish the project because you decided to check out early. You're here to bring your best. When you mentally check out early, you are not giving your best. This is your ego talking, telling you that you've done enough. You need to check that ego asap.
In all my years I the industry I see this far too often. One person finished his/her task and chills out for weeks after that. Sometimes months. This makes extra work for everyone else who have to cover for things that could have been taken care of, usually small things that no one wants to do. There's always pickup work that needs to be completed. Even if you're not good at it, don't sit back and ride the rest of the project out. If you're not good at a task volunteer for it anyway because usually you can get help, and it will be appreciated. If not, others will feel the guilt for having things cut or for the final product not being as good as they know it could have been.
Remember: you are here to be a valued member of a team and to support your teammates as you expect them to support you. You're also here to be of value to the company that hired you.
That being said, I don't recommend doing something WAY outside of your expertise.
Example: If you're an artist I don't recommend working on AI code. Stay within your field and help out the rest of the team. Especially if unforeseen circumstances pop up. Like a team member having to leave. There will be things that are out of your control that can't be wrangled with. So help out with things that are in your control. The more you help out within your expertise the faster you can honestly celebrate not only your accomplishments, but that of the entire team.
Remember: you'll take your lumps together and your victory laps together.