what does "Talk about cutting the tension with a knife,"mean?
“Hi,” she said, in a normal voice. “God. Talk about cutting the tension with a knife, eh? Guy’s always a perfectionist, but this is his first proper shoot since Lula died, so he’s, you know, seriously uptight.”

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Answer

We use "Talk about _____!" in colloquial speech to roughly mean "That's a perfect example of ____!"

The idea of cutting tension with a knife means that the tension in the air "was so thick that you could cut it with a knife".  The situation was so tense that you could physically feel it.