You've likely observed someone in writing, possibly a comedic character, start a word or statement but suddenly upon realizing their mistake, correct their Freudian slip, instances of letting the truth slip out, or slipping up in general, on the fly, midword or midsentence by shifting the slip up to a euphemism, less conspicuous word or some redirection, midword/statement, before accidentally saying the whole thing in error. I don't know if this form of interruption is to be written with an em dash or what exactly.
Example "Everyone's acting so strange toward me as of late" John Doe said with suspicion to which I, in what would become a botched attempt at calming his suspicion, replied "Oh, that's probably because they're getting ready for your surpri---fries ... 'Your super-sized order of fries,' ... that's what the guy at Generic Burger Joint was trying to tell me earlier. Darn, I forgot my fries at GBJ." super-sized fries being a phonetic substitute for accidentally beginning to say the full word surprise which would have, if said in full and without a redirect, spoiled the surprise that I'm clearly in on if I'd actually said more than the surpri part of the word surprise and not attempted to switch it to a substitute.
How does one actually write this midword correction in dialogue for a story. Also, though less important, what is it called if there is a name for it?