Pretty much any relationship between pain and depression that you can imagine has data behind it. There is evidence that they both interact. There’s evidence that pain impedes treatment of depression. There’s evidence that depression impedes treatment of pain. They are distinct but related problems. There is not really a stable prevalence of depression and pain samples because it depends on how the group was sampled, but in pain clinic samples, probably over half of patients presenting for treatment of chronic pain have major depression. In looking for depression, I would say that the most widely used and useful tool is probably the PHQ-9 Depression Assessment which is a self-report assessment that closely parallels the criteria in DSM-IV and DSM-V for major depression. A screening score of 10 or more has been accepted widely as indicating you need to know more. I think that one special problem in the pain population is that patients will attribute many of their depressive symptoms directly to pain which I think is fine, but they still count towards a diagnosis of depression. Sleep impairment, energy impairment, agitation, retardation, guilt, hopelessness, which all can be attributed to pain, still count for depression and I think indicate that you want to do some independent treatment of depression.
Antidepressants benefit people with chronic pain in three ways. First of all, if they have occurring depression, it helps treat that. Secondly, there are people who may not quite meet criteria for major depression but have sleep disturbance, and the antidepressants are a far superior treatment for sleep disturbance to the benzodiazepines that also amplify opioid risks. Thirdly, there is a long and robust literature showing the antidepressants function as analgesics. That literature is strongest in the case of neuropathic pain but in musculoskeletal pain, there are also good studies. In general, antidepressants with noradrenergic reuptake inhibition, otherwise known as the SNRIs or the tricyclics, are more effective analgesics.
I think you want to frame the depression as a reasonable response to a severe pain problem. You need to acknowledge that patients’ pain is real, that the depression is a significant complication of a pain problem that warrants additional treatment. If your patient believes you’re addressing the physical aspects of their problem as well as the mental aspects, they’ll be much more open to the depression treatment than if they feel like they’re being written off and sent to the shrink for a problem that’s all in their mind.