Doris said the advice from the fly-half, who skippered the Irish from after the 2019 Rugby World Cup to their quarter-final defeat in the 2023 edition, was succinct: "Be yourself."
Doris gets the opportunity to captain the team due to Peter O'Mahony (34) having a calf injury sustained in the sublime 38-17 whacking of France in Marseille last week.
O'Mahony too has been generous with his advice to the back row forward who has been outstanding since head coach Andy Farrell called him up in 2020, although his first Test lasted just four minutes as he went off with a concussion.
Doris, a psychology graduate, said he has not just used his time on the pitch performing but also observing how Sexton and O'Mahony captained the side.
"I was even on the phone to 'Sexto', to Johnny, there during the week," Doris told reporters on Saturday.
"The tendency is seeing Pete and seeing Johnny doing such a good job in how they do it and thinking how can I emulate that.
"But their message has just been be yourself, being authentic is the best way to lead and finding my own way to do it, which is challenging at times but it's been good."
Doris, who captained the Ireland Under-20 side, said Farrell had taken him aside early in the week to give him the news.
"I found out Tuesday afternoon," he said.
"We were back in camp Monday night and then our first training day was Tuesday so Faz (Farrell) gave me the heads up then that Pete was going to be out and I was going to take his role up for the week."
'Exciting prospect'
Doris said he had experienced a whole host of emotions.
"Delight was the initial feeling but also nerves and excitement and finding my feet in the new role for the week," he said.
"There's been a bit of mixture of all that but it's pretty cool honour to have for the week."
It is not the only change in Doris' status for the match as he also has to move to flanker from No. 8, replacing 2022 World Player of the Year Josh van der Flier as Jack Conan takes his position.
However, Doris is not in the least bit flustered by the switch even though he says he "prefers No. 8".
"I'm pretty familiar with the seven role from covering it," he said.
"If it's me and Josh with Leinster or Ireland, I'll be the back-up seven.
"I've been in that role and practise it in training quite a bit, swapping in and out with him over the last number of years."
Despite rave reviews for the manner in which the Irish dismantled the French - it was a record away win for them in France - Doris says there are areas they need to improve on against an Italy side that pushed England hard in an opening 27-24 defeat.
"The two most noticeable was our discipline, we conceded 14 penalties, quite a few in the second half as well, so that's a more obvious one, and then our scrum didn't fire as we'd like it to," he said.
"The challenge this week is to back up last week's performance with an even better one across all areas. It's a pretty exciting prospect."
