Reasoning behind a cast-member's credit status?

What's the reasoning behind a cast-member's credits status? Sam Atwell was only added to the permanent cast 3 months after his arrival (despite dominating the storylines from his first day), whilst other (arguably lesser - in terms of plot significance and screentime) characters were added almost immediately. Was it a question of keeping viewers guessing as to the outcome of that arc, or (in true Casablanca style) did you genuinely not know yourself, and if so, at what point (was it ratings driven or did seeing some scenes play out just 'click' with TPTB?) did you decide 'yep. He's working out.'?
Asked by Mona Rashid

The credits thing can be for a few reasons. Firstly, we only want a certain number of characters on the opening credits. Also it costs to redo them so we tend to do it in batches - a certain number in and a certain number out at the same time. A new regular might come in at a time when were not ready to do that process, so they stay on the guest cast for a while until it's the right time. Note how quickly recently Josie and Dan went in to the front credits. One reason for that is that they came at a time when a number of characters were leaving and we were re-doing the front credits. Another scenario is that some characters come in as long-term guests, aren't seen as regulars, but act their way in to popularity and long-term contracts - Sam is a perfect example of that. Kane was a short-term villain who became one of the best creations the show has ever had thanks to Sam's performance and some very good storytelling.

 
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