I was well aware of the fact that once you appeared in Doctor Who as something else, you were ruled out for the part of the Doctor: that was a kind of well known thing in the business.
Meaning of the quote
The quote explains that if you appeared in the TV show Doctor Who playing a different character, you would not be able to get the main role of the Doctor later on. This was a well-known rule in the entertainment industry at the time. Once you were cast in another part, you were no longer considered for the lead role of the Doctor.
More quotes from Colin Baker
If you turn down work because you are frightened of getting typecast, you’ll never do anything good.
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