Yeah... I was trying to avoid the issue of asking about variations of "said." My question was more about how to make it grammatically correct. I realized after I saw some of your responses that I forgot one other situation. The line-- William was surprised by that. “Really? They can do that?” Contains two real sentences that are able to end in a period. But what about something like... William shrugged and gave a nonchalant, “It wasn’t difficult.”
If people are giving you a critique that says you should replace "said" with a bunch of other tags (I'm not talking about where they want you to eliminate a tag or use a beat), they're likely to be inexperienced writers in their own right and probably not the best people to be giving a critique.
A beat, tatala. A beat. This example is a little tortured, but syntactically I think you've got it right. Gave a nonchalant is syntactically equivalent to nonchalantly said.
Round about draft 74 we/or you might write: "They can do that?” said William. [Think 'white space power' or properly, power of the white space on a page, and picture me imagining William's handsome features. You don't need to tell me everything, because I am telling myself.] And always read your fine dialogue aloud - to yourself, and pets... At some point when reading aloud - sense, beats and rhyme hangs loose [in the English meaning of such] without a tag, so shove in a 'said' at this junction. Produce eight chunks of speech, two or three 'said' moments, like a sweetener really, or invisible sign posts - grasp the concept on the invisible highway.. heh :/