I'm not a layer so I'll leave it to the lawyers here to discuss common/ not common and fair or not fair practice. That said, I find it odd that Fina and Eshbach show up for Paterno's interview. I find it more odd that state prosecutors are 'interviewing' people like 30- 40 minutes prior to them giving grand jury testimony.
Is this common? I would assume witnesses would have been interviewed long before they were ever called to testify and so why would prosecutors be interviewing them again right as they go into the grand jury? My suspicion is to lead witnesses and make sure certain points are stressed and emphasized right before the witness steps into the grand jury room. Seems close to witness tampering but I'm not a lawyer to know how common or appropriate the practice is.
As a side note, I also get very suspicious now when I see an AG (or DA) call a press conference where they have big poster boards of the accused with inflammatory phrases in bold letter above a mugshot. If someone has done something wrong then charge them and take them to court. Do the talking and convicting in the court room. The PR nonsense of trying to convict that person in public opinion in a press conference (or prejudice a pending jury) before they've ever had their day in court just seems unethical to me.