"When he hugged me he was shaking and giving that loud grown man cry," Johnson wrote on Instagram. "Not only will I never forget this day, but stuff like this will always be the best part of fame."
Johnson said Fallon and he were shooting the skit at Universal Studios in Orlando.
"As fate would have it, out of the thousands of people walking around Universal, the show's producers chose Michael Nevin and his wife Jessica to take a fake picture on the Tonight Show stage," Johnson wrote.
"Not only were Michael and Jessica on their honeymoon, but he's also been one of my biggest fans since he was 5 years old."
And the best part: Nevin has three tattoos dedicated to "The Rock."
"This reaction was priceless and I'm still shaking my head at the chain of events that had to happen for this one moment of awesomeness to go down," Johnson wrote.
"Thank you universe. Thank you fate. Easily one of the best parts of fame."