Hi!
Of course, it is ALWAYS okay to take a moment to think through your answer! Multiple times during an interview, if necessary!
This is even something that is tested in MBB interviews - whether you have the maturity to order your thinking before blurting out immature thoughts. Because this maturity is exactly what is expected of you in a client situation! From the perspective of a Project Leader, there is nothing more dangerous than a young colleague making undeveloped statements in front of clients - there is no quicker way to destroy creditbility of the entire team.
Cheers, Sidi