Welcome to Gaia! ::

It depends. The main thing of importance is to not simply recite something by rote because it's the rote. You need to feel and understand what it is you're saying and doing. If following a script helps you do that, follow a script. If it dosen't, improvise.

I don't script. I don't need to script, though. I have enough of a background in ritual structure and in magical systems to be able to do things on the fly. If you're just starting, a structure can help you to learn.

I strongly agree with what has been said so far; it varies from person to person. Some traditions require that you are and some don't, or don't really specify ether way.

I personally am not a Ceremonial (one uses a lot of tools and strictly followed pre-written rituals) witch. I actually work somewhat chaotically. There's somewhat of a structure or idea and always a reason for the ceremony/ritual, but the exact direction isn't known until I get into circle to feel things out.

I sort of metaphorically speaking, pack everything I'll need for the trip, see my destination, then just start running wildly letting my instincts guide me. I still can see the destination in my mind and know where I'm going, but where exactly my footsteps fall on the way there, isn't known until it's already been made. If that makes any sense.