Having toured twice with the Lions, and scored two tries on his last appearance in the third test triumph in South Africa four years ago, the 36-year-old Welsh legend is nothing other than a proud Lion.
"Rob phoned me in my hotel room at about 11pm on Saturday night asking if I would help out the Lions by playing on Tuesday. There was no way I was going to say no," said Williams, who was due to travel to Brisbane this week to work as a summariser for talkSport.
"I'm happy to be here and I'm enjoying myself. Four years ago in Johannesburg was a great day for me and the Lions. It was unfortunate we lost the series, but I came off that field immensely proud thinking that was the last time I'd wear the Lions jersey - and I've thought that for the last four years.
"To be given this opportunity again is really a dream come true - and completely out of the blue. I knew nothing about it.
"I appreciate the Lions have had a lot of injuries during this tough tour so far and if me playing tomorrow helps the Lions to recover, and do whatever it is best to prepare for Saturday and win that game, then I am very proud to be a part of it."
Williams shook hands with team mates new and old at the North Sydney Oval this morning and started the process of getting to know names, calls and game plan for Tuesday night. He also got a chance to meet his new wing partner, London Wasps' rising young star Christian Wade.
While the Welsh wing wonder can boast 60 tries in 91 Tests for Wales and the Lions, the 22-year-old Wade has just earned his first England cap. Even so, Williams is predicting big things for him.
"I've paid close attention to Christian and I've spoken to Stephen Jones about him. He is a very talented player, new on the scene, but already raising eyebrows," said Williams.
"He is a massive talent and I enjoy the way he plays. He is a very aggressive runner, obviously very fast and he enjoys taking defenders on - my kind of guy and I am looking forward to playing with him. He is more than capable of showing what he has got tomorrow and being part of this Test series."
While Williams came from Japan via Hong Kong, Wade flew in from South America to join up with the Lions. Williams has signed on for three days, but Wade will remain with the tour party.
His big chance comes at the end of a season when he became the first player to do the double at the RPA Awards dinner by winning both the 'Young Player of the Year' and the 'Players' Player of the Year' titles, finishing as the joint top scorer in the Premiership and earning his first England cap.
"It has been a bit of a rollercoaster few days and week. I've just made my debut for England and now I'm out here," admitted Wade after his first training session with the Lions.
"I'm happy to be here and I just want to get stuck in. I'm starting tomorrow and that is the first step, but if I do get handed the opportunity to play in the Test then I will gladly take it - it would be the proudest day of my life so far.
"It is a massive thrill for me to play with Shane. I have watched him from a distance, and tried to learn from him, but to be able to play alongside him is beyond my dreams. I never got the chance to play against him and to be able to play with him is taking it to the next level."
Wade, Williams and the third emergency recruit, Saracens and England centre Brad Barritt, will spend the rest of the day reading and learning the Lions' play-book as they try to play catch-up with the rest of the squad.
The preparation time for everyone concerned has not been ideal, but all the players realise there is an unbeaten record to protect in Canberra on Tuesday. They realise it is going to be difficult, but not impossible.
"If training is anything to go by this is going to be a very difficult exercise. We've got to learn all the new calls, but we are professional rugby players and we've played alongside the majority of these players before," said Williams.
"We have to do our homework tonight to make sure we are mentally prepared for this game because we don't want to let anyone down. We want to give 100 per cent and we don't want to let ourselves or anyone else down.
"My last game was on Saturday in 35 degrees, although it was not at the same standard of tomorrow night's game. But I have kept fit and I've worked just as hard in Japan as I have throughout my rugby career.
"Hopefully, it is not going to be too much of a shock to the system. I have played enough professional and international level games to know what to expect tomorrow and if I felt for one second I wasn't going to be ready or good enough I would have told Rob Howley where to stick it.
"To begin with it is all about communication. It is not going to click straightaway - it is about learning the calls, understanding the patterns and finding out how players play.
"It won't come together straightaway, and it didn't in training, but slowly as the training session went on we spoke a bit more and we were more comfortable together. It is going to be intense tomorrow, but we are going to work as hard as we can in the next 24 hours to make sure we are ready to do the job."