Here is the scenario: in a collaborative effort, our group was to work with another group/professor to write a paper on some work we had previously done. I, a grad student, am to be the main author/architect of the paper. But, and this was said explicitly by him and my own professor, I was to write it with lots of help from the collaborating professor, and under his guidance and direction.
In other words, I would talk to him, get direction like "you should focus on this topic for the paper" or "don't pursue this, it won't be interesting to reviewers", and I would do the actual work/research/writing.
However, since making this agreement, months ago, he has gone basically incommunicado. I'll send him an email and not get a reply for literally a month. I've asked to set up a short (<1hr) meeting, at basically any time of his choosing, and just no response. Several times, too.
Let me be clear about this too: I'm not really that upset about the lack of communication itself (though, I'll say this single thing about that: when he is actually in person with me, he is constantly checking/answering his email on his phone, so I know he does do it, just apparently not with me). I'm not dying to write the paper, it's not a cutting edge thing that will get scooped, and I have several other projects. It's a little annoying in general that he has been flaky, but I'm plenty busy with other stuff, so again that's not too bad.
Here's what my question is about. I both suspect, and from my short interactions with him I've had in the past few months, that he is going to ask to see what work I've done on it since he became uncommunicative, to which the answer is "minimal". The reason is that I am busy with other projects, and because he wasn't doing his part of this, I didn't know which direction to take. I could certainly speculate and choose one, but that is very risky (timewise) for me: if I spend several weeks pursuing something and it turns out he doesn't think it's a good fit for the paper, welp, I just wasted several weeks. And that has happened to some extent.
Because it seems like I wasn't clear about this initially, I'll say it explicitly here: my goal is not to attack him, or bring this up. If he seems fine with the (lack of) progress made, then I'm very happy to leave it be and just continue on. But I strongly suspect (and he seemed to say) that he expects me to have done more. For this question, assume that the professor will ask why more hasn't been done on the paper, so the subject will come up.
So, how do you say this tactfully? You can't really respond with "because you were uncommunicative, I didn't get very much done, because it very possibly could have been for naught", because that's pretty accusatory and will ultimately not help solve the problem.
Is there a good way to say this?