When Gavin and Stacey have their first dance, look carefully and you can see Nessa sat at a table behind them with other people. Seconds later, the camera moves and focuses on Nessa standing at the bar alone.
On the episode where Smithy goes AWOL after learning he's to be a father, Gavin, Mick and Bryn go searching for him in the car. They decide to visit the drive-thru where Rudi works to see if she has any idea. Before Rudi comes over to the window, there is a big woman who calls for her. This woman is James Corden's (Smithy's) sister in real life. She's called Ruth, which is also Smithy's sister's name (Rudi is her nickname).