Sean Penn went full fiesta after scoring his face-to-face with El Chapo ... boozing with Kate Del Castillo at 35,000 ft.
The party in the air didn't begin that way ... our sources say Sean and Kate were "somber and nervous" when they boarded the Hawker 900 (below) at the Van Nuys Airport October 2nd. We're told they said virtually nothing during the flight to Guadalajara.
The flight back home was raucous. After the El Chapo interview, Sean and Kate boarded the same plane in Guadalajara. They were alone on the plane and broke out the booze, laughing and hugging in full celebration mode.
When they landed in Van Nuys they had to check through customs but they brought nothing back and cleared without incident.
What we don't know is how they answered the customary question -- What was your purpose of going to Mexico?