A dialogue “tag” is the narrative that indicates who spoke the line. Phrases such as “he said” or “Marsha yelled” are considered tags. When writing dialogue, think of the tag as part of the same sentence as the line of dialogue it follows: “I’m tired. Forget about the party,” he said.
So what happens when you add a question mark to the mix? Essentially, the question mark replaces the comma that connects the dialogue and the tag:
“Is there another loaf of banana bread?” she asked.
Notice that the “s” in “she” remains small. The dialogue and tag are still acting as one sentence, even though that question mark is there.
When you follow a line of dialogue with an action or other narrative, they function as two separate sentences, with the first word of each sentence capitalized:
“I’m tired. Forget about the party.” He wrenched open his briefcase, looking for his cell phone.
The same is true with questions in dialogue:
“Is there another loaf of banana bread?” She peered over the counter, trying to see behind the baker.