Three years later, Williamson says the "shock" of training with the national squad has worn off after he was called up again during the recent Six Nations.
"I feel a lot more confident going in this time," Williamson told BBC Scotland. "I know a lot of the Glasgow guys a lot better. Last time was a bit of shock and that week went really fast. To go back and assert myself a bit better was a real positive.
"It was a really good experience with a few of the other younger boys. It's really cool to see their prep for the week and how that's different to ours and what to take from that."
This has been Williamson's breakthrough year at the Warriors, making his Glasgow debut last December and going on to make nine appearances so far, three of which have been starts.
Fellow lock Alex Samuel and Euan Ferrie have also trained with Scotland in recent weeks after impressing for Glasgow.
"It's good fun, it makes the situation a bit less stressful when you've got a couple of mates in with you. To go through and develop together is awesome and hopefully we can come back and take our learnings into the Glasgow games over the next few weeks.
"We're focused on building a squad to try and push for trophies - and you need to have a strong squad, not just a 15 or a 23. It's a positive that [Franco Smith] has given a lot of the young boys opportunities.
"A lot of them are taking those opportunities - the likes of Ben Afshar at start this season, or Gregor Hiddleston as well, they've both taken their opportunities and that's credit to them."