If you’re a lead on a series you have a particular responsibility.
Often, the biggest problem is learning your lines scene after scene, day after day, week after week.
You’ll develop your short-term memory.
The pressure on everyone is to shoot quickly. On most shows there are few takes although lots of coverage. Doing your best in each take is enough and if they move on then accept it was good enough. There is no time for second-guessing as a lead. The next scene is coming up.
The producers expect you to deliver the same tone, mask, behaviour episode after episode and season after season. The advertisers have invested in that. To do that you’ll nearly always be dropped in instead of having to work hard to get in over and over. That’s too tiring.
All actors are looking to find the most space they can to give them freedom to play.
As a series lead you’ll find your bits of space here and there on set to put together to make your whole space. It could be by always checking the writing with the show-runner or writer on set; questioning a part of the blocking; asking for one more take, etc.
TV writing doesn’t always make sense but rather serves the needs of the show as dictated by the producers. Play each scene on its own. There is usually no arc.
In the end, dealing with fatigue may be the biggest task.