Given that the whole experience hinges on being drawn into your patients' lives through the stories they tell, I'm happy to report that the acting is really well done and professional. Nathan's laid-back and resigned, while Bryce is trying to sound calm but bursting with nervous excitement, and Claire is stuck-up, wealthy, and used to treating people like servants. It would be easy to go over the top in a situation like this, but all the actors manage to show emotion without chewing the scenery too much. (Apart from Claire, who like many aristocratic types seems to have been taught from birth to keep a stiff upper lip and not to show what she's feeling.)
The soundtrack, unsurprisingly, is there to provide a steadily-mounting feeling of ambient apprehension through languid chords rather than play cheery tunes. As such, it does a solid job, rarely being consciously noticeable but adding to the feeling of slowly-mounting dread.
A pane on the left of the screen allows you to switch from patient to patient with a click, as well as browse all the questions you've asked so far. Clicking on an already-asked question replays the answer clip with a sepia tint, and asterisks are used to highlight questions with answers that are worth following up on, indicating that pressing the patient about something in their answer will lead to a new response. One green asterisk indicates that there's more to say but it's not vital to the investigation, while two amber asterisks identify avenues you will need to pursue to make progress. The patient names are also marked with coloured dots, showing whether you'll need to get more out of them to proceed (red), you've got enough but not everything (amber) or they've told you everything they can (green). Once all patients are at least amber, you have the option to move on to the next stage. Get to the end of the last act, and you have the opportunity to accuse the culprit. (Not to worry if you get it wrong, though: accuse someone innocent and they'll be upset with you, but you'll get another go.)
While the brief tutorial encourages you to embrace your role by asking natural questions, such as "How are you feeling?" or, "How do cats make you feel?", you can also just stick to keywords, such as "cats". There's no fancy Infocom-style parsing going on here, so if you do ask questions the parser will just pick out keywords, like its distant ancestor ELIZA. For the most part this works pretty well, and there's even a neat explanation (involving verbal mirroring) for why you need to stick to pulling out exact words and phrases from your patients' responses. However, it's not perfect and every so often I ran into spots where I struggled to be understood.
For example, at one point a client tells you that her boss "accused me of stealing." I responded with, "Why did she accuse you of stealing?" and got a blank look and a confused reply. (Each person has a set of canned responses for questions they don't understand.) As it turned out, to get her to talk about it I had to use the phrase "accused me of stealing" exactly, despite that not making sense as a question. Likewise, "How are you?" is a good way of starting the conversation, but "How do you feel?" is greeted with confusion. If you keep running into issues, the subtitles can be set to pick out keywords in italics, though only to get you to essential responses. It's also worth noting that the developers seem to be responding to issues like these through a series of updates. In the end, though, while the idea of being able to ask natural questions and get intelligent responses is neat, these little hiccups meant I tended to fall back on terse phrases instead.
Speaking of responses, your patients have a lot to say. Each of the five acts includes about as many possible responses as the whole of Her Story, and it took me around eight hours just to hear a bit more than half of them. (The title screen shows your progress within the act and how many of the available responses you have found, for the completists out there. I found that typically about 60% of the possible answers were needed to progress.) Add to that the fact that some of the responses depend on the narrative branch you've chosen and so won't always be available, and this must have been a big project.
The downside of all that dialogue and all those branches, though, is that it never really feels like there's much to the underlying plot. You will find out what happened to Dekker (at least in the version of reality you steered yourself into), and uncover enough clues to pin down whodunnit, but it's mostly a series of loosely-linked stories about his patients rather than an Agatha Christie-style mystery. Thankfully, I found these stories to be gripping, coming to genuinely feel like I knew these people, and I was glad of a brief wrap-up at the end that detailed what they went on to do. It was odd, too, not to feel encouraged to go one way or the other: helping them and giving in to the same madness that took Dekker are equally valid (and interesting) options.
The developers haven't shied away from hard science and sci-fi, either. There's talk of everything from parallel worlds to quantum suicide, from chaos theory to Cthulhu. Not just that, but they're given coherent explanations that encourage you to believe the world may just be that weird. (As a sometime-quantum physicist, I've often wondered about that!) I was left with a lot to think about, although it's also perfectly possible to dismiss all of it as just psychosis and keep your feet firmly rooted in the "real" world. Except, where would be the fun in that?
The Infectious Madness of Doctor Dekker is ambitious and atmospheric, leading you steadily down into a nightmarish world where anything is possible. The diverse, engaging characters and branching narrative draw you in, even if the actual mystery is something of a side issue. While it can't quite achieve its lofty goal of allowing you to probe your patients using natural questions and consistently get thoughtful video responses, it comes close enough to give you a powerful feeling of being there, listening to their problems and gradually getting sucked into their unreality. The murderer could be any one of them, and even beyond that they each have choices to make, giving you ample reason to come back for another session, if you dare. It's a unique and memorable experience, and definitely one to play late at night with the lights off and the wind howling outside.