In the mid-1930s Hollywood was regaining its confidence after the difficulties of the talkie transition. Although all the technical problems of sound had been solved very quickly, it took longer to resolve the questions of how talking pictures should look, how they should be structured and how they should be acted. The Informer is a key picture in that it shows the extent to which wordless moments can convey story, asserting the power images without ignoring the necessities of sound and dialogue.
This is not to say the Informer is truly a throwback to the golden days of the silents. For one thing, many silent pictures were not so purely visual in their narrative, and were overburdened with title cards. But what the Informer has is the self-assuredness to extend moments between dialogues, to focus on reactions more than speeches, and to let shots play out simply for atmosphere.
Director John Ford, for all his capability, was a filmmaker who appears to have put in effort in proportion to how interested he was in the material. If he thought a story was silly, he just did it half-arsed. Luckily the Informer, with its depiction of community, honour, working class life and most importantly Irish setting, was everything Ford loved, and the result is one of his finest works. In it, Ford only really employs too kinds of shot. The first is of places  the Dublin streets shrouded in mist and darkness so their furthest depths cannot be seen; dingy interiors where the walls and ceilings seem to press in on us. The second is of faces, striking close-ups against plain backgrounds, usually without dialogue, focusing us upon the inner conflicts of these people.
Lead man Victor McLaglen fits perfectly within this character and this manner of filming him. McLaglen's performance does not look like much, being as it is about 90% drunk act. But the other 10% is heartfelt emoting, as here and there his Gypo Nolan has what alcoholics refer to as a moment of clarity. With such performances are Oscars won. McLaglen is backed by a spot-on supporting cast, among whom there are no weak links. In particular it is nice to have Donald Meek and Una O'Connor, usually only seen in comic relief roles, playing straight dramatic parts for once (although Meek's appearance does contain one or two jokes, the tone of the scene and much of his manner is serious). Not only do these two deliver incredibly deep performances, their familiarity to most viewers as comedy players gives an added note of poignancy to their part in this tragedy.
RKO, who produced the Informer, were perhaps the most adventurous and willing to take risks of all the major studios. Thanks to this, we are able to see a dismal story with a despicable anti-hero at its centre, which could easily have been a clunky, over-earnest mess, instead filled with a moody atmosphere and depth of character which keeps us watching and draws us into its world.

Mona the vagabond lives on the fringes of French society, in a life without meaning, purpose or direction.
I watched this because of all the stellar reviews, but I'm afraid I must have missed something. The character of Mona has little or no personality while drifting through life being rude to people, getting high and contributing nothing to anyone's life. She's not interesting or exciting. She's just useless.
I've seen and known enough people like that: there is no secret meaning to what they're doing. They are just lazy bums. I wouldn't want Mona anywhere near me, as she tends to steal anything that isn't nailed down and leave her friends in the lurch. Sure she's enigmatic - because there isn't anything to her. Lots of junkies, winos and bums I've seen are enigmatic; I wouldn't want to see a film about them either.
Possibly there is something there that I totally missed. Otherwise I'm assuming that all the reviews are from people who assume anything done by a French female director is high art.