during credits in tv series' how do they decide which starring actor has "as (character's name)" & who doesnt?

Question

i noticed once on Charmed that alyssa milano is credited as just alyssa milano while holly marie combs has "as piper halliwel" written after her name! they've both been on the series for the same amount of time so it can't be anything to do with seniority

Answer 1

Billing (how the name appears) depends upon the negotiations that were done before the actor agreed to be on the show. Some people like it one way, and others like to have it another way, and some people don't care at all.

There is a lot of negotiation that goes on for trivial details like this.