"I've got a chance and I've just got to take it," said the 24-year-old forward, who has so far made two substitute appearances for his country.
"It's like a dream - I have gone from playing for my club to making Wales' starting line-up. But it will be very real tomorrow and I've got to go out and perform.
"I wasn't really expecting to start, especially as Humphs has come in and done such a great job, but he has picked up an injury, and I've got to go out and play now."
Although Humphreys' contribution on the field will be sorely missed against Scotland, his continued presence in the Wales camp has given Williams a timely boost.
"Humphs has helped me a lot. I am still learning, and he has given me pointers with his experience," he said.
"He's always talking to me and helping me, and it is great to be able to learn off someone like him."