If the quotation precedes the reporting verbA part of speech that expresses an action or a state. E.g.: John seldom plays tennis., we can use full inversionA structure in which a verb or an auxiliary precedes the subject. E.g.: "Here comes the sun." and place the verb in front of the subjectThe person or thing that performs the action of a verb or that is described by a verb. E.g.: Andy has never been to Brunei. ("Andy").

"I couldn’t be happier at the moment," said Johnny.

Note that the following is also possible:

"I couldn’t be happier at the moment," Johnny said. (no inversion)

If the subject is long, inversion is usually used:

"What's the time?" asked the man in the black jacket.

But if the subject is a personal pronounA part of speech that refers to or stands instead of a noun. E.g.: Who do you think I am? ("you", "I"), no inversion is used:

"What’s the time?" he asked.

Full inversion after place adverbials
Full inversion Partial inversion