The Rovers midfielder spent over two seasons with the Vicarage Road club, and has fond memories of his time with the Hornets.
"It is good playing against your previous clubs, I was there a few seasons ago," said Williamson.
"I think I was there for two-and-a-half years, it was a good time, a good club, and we nearly got into the Premier League with them.
"So it will be interesting going back to play there. You always want to do well when you return to your previous clubs but it is just like any other game really."
Rovers are in high spirits following their last two encouraging displays, and the midfielder knows that it is important to carry that on this evening.
"We've had two good performances now and we need to follow that on, carry on doing what we are doing," added the midfielder.
"We are going to go into every game now looking for three points. We can't sit back and play for draws, we know we need to push on now and get three points now to get ourselves up the table and give ourselves a chance of the play-offs.
"There's no pressure on us, we just have to do what we know we can do and just win games.
"We know it isn't going to be easy, it is a tough league as we all know and it is possible that we can get there.
"We've set our standards, that is our benchmark now. We've to take what we have learnt from the last two games - Huddersfield and Leicester - take all the positives from it and put it in the remaining games."
Tickets for this evening's match will be on sale at Vicarage Road from 6.30pm, cash only from the away turnstiles.
Tickets are priced at £26.00 Adults, £15.00 Seniors over 65 / Students aged 24-20 with valid NUS Card / 16-19 year olds and £10.00 Under 16’s.