Bears outside linebacker Sam Acho was there when it all began on the field for quarterback Mitch Trubisky in coach Matt Nagy’s offense. It was April 17, the first day of an extra three-day voluntary session given to Nagy as a new coach.

Of course, it was Acho’s job to stop what Nagy was doing. That was the case for him and several other healthy defensive starters, who have faced that offense in every practice since that day.

‘‘They’ve grown so much,’’ Acho said.

In other words, Acho knows what’s coming from Trubisky. He knows what has developed and blossomed under Nagy. And he knows what their combination could mean for the Bears this season.

‘‘They have a lot more from when we started in April to where we’re at now,’’ Acho said. ‘‘It’s almost like April and [organized team activities] were Phase 1, and then training camp you’ve got Phase 2. And then I’m sure there are going be other phases and iterations.’’

Acho said it’s an ‘‘evolution’’ to be excited about.

‘‘It’s exciting to see what that offense has the potential to do,’’ he said. ‘‘I see it in practice every day.’’

A different attack

On Feb. 1, running back Jordan Howard aired his grievances about former coach John Fox’s brand of football on national TV.

‘‘Last year, everyone knew what we were going to do,’’ Howard said on ‘‘Good Morning Football’’ on the NFL Network. ‘‘They knew what was coming every play. It was easy for them to stop us.’’

Not now. Not with Nagy.

‘‘It’s a multiple offense,’’ safety Adrian Amos said. ‘‘Certain things, when they have a lot of different options, sometimes it’s hard to really pick up tips and things on them. It’s basically going off what the defense does.’’

But it’s not a take-what-the-defense-gives-you approach. No, Nagy’s offense wants to take everything. The offense plans on doing that through a wide range of looks. It’s different run-pass options, the zone-read option, unique personnel groupings and formations and more.

Acho said Nagy’s offense tests your eyes but also your ‘‘understanding of defenses and of offenses.’’ During the preseason, the Bears used RPOs that featured different quick screens out of a bunch formation.

‘‘Let’s say I’m the outside linebacker head up on that bunch, and I think it’s a run,’’ Acho said. ‘‘I just dash and go in for the run, and they throw the screen. Now we’re out of luck.

‘‘That’s what I mean about testing your eyes. They probably have plays out there where they’ll be in a bunch look, and you think it’s going to be a screen and it’s a normal route. Like Kevin [White’s] touchdown [against the Chiefs]. There is a lot that our offense can do, which is what gets me excited.’’

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The Bears can do a lot within a play. The RPOs that include screens are only one example. There are a lot of moving pieces before the snap as Nagy’s offense seeks out coverages.

‘‘For us, it’s keeping your eyes in the right spot and really trusting your technique,’’ safety Eddie Jackson said. ‘‘It’ll help you. But [Nagy’s offense is] difficult.’’

Defenses must be prepared.

‘‘It keeps you on your toes, and it keeps you in your ‘book,’ ’’Amos said. ‘‘You’ve got to make sure you stay on your keys and different things like that because it challenges and stretches different coverages, depending on what you’re in at the time.’’

Filled with upgrades

As the Bears’ last line of defense, Jackson and Amos have dealt with the personnel upgrades on offense more than most defenders. They know the speed of the offense can kill.

‘‘It’s most definitely faster,’’ Jackson said. ‘‘I’ve been talking about it since camp. It’s a lot faster. You’ve got guys who are stretching the field vertically and that can run. That’s one of the best things [for them] and one of the toughest things for a defense, especially in the secondary, when you’ve got fast guys that can run and make plays down the field.’’

The arrivals of receivers Allen Robinson, Taylor Gabriel and rookie Anthony Miller and tight end Trey Burton changed the competitions in OTAs and camp, particularly in one-on-one drills. It was better. It also was imposing to see them on the field together in team drills.

Trubisky wasn’t checking down much in OTAs and camp. Following Nagy’s orders, he took shots down the field. And then he took more shots. Nagy repeatedly has said the aggressive approach would continue in the regular season.

Part of the challenge for defenders is how Nagy features his receivers and tight ends. He’s always looking for mismatches. His variety of offensive talent led to a variety of personnel groupings, formations and route combinations. With running back Tarik Cohen and tight end Adam Shaheen added to the mix, there was a lot to digest and handle.

‘‘They have routes that most teams don’t run,’’ Acho said. ‘‘You have to be ready. Even though you’ve never seen that look before, you still play your responsibility.’’

It took time for players on offense to understand their own responsibilities, but several defensive players said that’s not the case anymore. They can tell by how much faster the plays are being run.

‘‘They really know what they’re doing,’’ inside linebacker Nick Kwiatkoski said. ‘‘They know what the ideas are, what the scheme is. And you’ve got guys lining up everywhere, which makes it hard on defenses.’’

Nagy’s offense is a far-from-finished product. To him, it’ll be like that for years. But members of the Bears’ defense still see something that will produce this season.

‘‘It’s really just the chemistry,’’ Jackson said. ‘‘They’ve started to build chemistry with each other, getting used to the new faces that have been put on our offense. It’s looking very good.’’

It starts with Trubisky’s comfort level in it.

‘‘They’ve come a long way,’’ Kwiatkoski said. ‘‘You can see how comfortable they’re getting. It’s the plays they’ve run, how they’re building off each little thing. It’s not easy to guess plays. Things were limited coming into OTAs. Just that growth from there, it’s been fun to watch.’’

EXTRA POINTS

Talking it out

During the course of five preseason games, Bears coach Matt Nagy learned what works and what can improve as he runs his team on the sideline. As he expected, it has been an adjustment for him.

He got accustomed to calling timeouts and got a feel for challenging plays, which starts with communicating with his coaches who aren’t on the sideline.

Nagy challenged two plays Thursday against the Bills, winning one that overturned an eight-yard catch by Bills tight end Keith Towbridge on the opposite sideline.

But for Nagy, it also was learning how to focus on his call sheet and his own game plan while still paying close attention to his defense and special teams.

‘‘The communication process with the offensive staff and myself on how we’re going to do that, that was one of the more valuable parts of the preseason,’’ Nagy said. ‘‘We tweaked some things on how we did some of the internal stuff. None of it was major, but it’s been better the last couple of weeks. So that’s a positive going into it for the first time. That’s probably really the biggest thing.’’

Wims catches on

Rookie receiver Javon Wims played six snaps, all on special teams, against the Bills. But it was a good sign for him. If Wims is going to be a reserve receiver early in his career, he needs to be able to handle a spot or two on special teams.

‘‘It’s cool,’’ said Wims, who didn’t play special teams at Georgia. ‘‘I’m just trying to find a role to help this team out, so anything that would help.’’

The Bears didn’t need to see what Wims could do as a receiver against the Bills. He already had impressed them with his 15 catches for 227 yards and a touchdown.

‘‘I showed that I can make plays whenever I got the ball,’’ he said. ‘‘That’s all I pretty much wanted to do [in the preseason].’’

When asked about the receivers as a whole, Nagy said Wims is making fewer mistakes when it comes to his splits, alignments and adjustments against certain coverages.

Wims’ touchdown catch against the Chiefs is an example. It came on a corner route that veteran quarterback Chase Daniel checked to at the line.

‘‘I just stopped thinking and started reacting,’’ Wims said.