Farrell, who collected eight caps for England, highlighted the qualities of Twelvetrees and his potential to fill the role of a creative international centre.
"He's got talent across the board," said the former Saracens centre. "I've been very impressed.
"We know that he's played a lot at ten but what he has brought into his game in the number 12 role is the ability to get over the gain line and take the ball up and be aggressive in defence.
"He's a big boy Billy, with a big presence and he's got all the tricks of the trade that a ten and a ball-playing 12 could have.
"But he is now developing the balance of a game that you need to play at 12 in international football."
Twelvetrees has excelled at Gloucester since his move from Leicester Tigers last year, and Farrell insisted the new squad member must grab his opportunity with both hands.
"He brings another good head as far as game management and kicking skills are concerned. We've seen him improve massively because of playing week in week out," added Farrell.
"We're interested to see whether he can transfer the form from his Gloucester week in week out games into the international set-up.
"The key is to be able to go from what you are used to week in week out to expressing yourself straightaway.
"It's important to make an impression on your team-mates straight away at international football."