Richard Engel, Chief Foreign Correspondent for NBC and a dear friend of Rachel's, tends to provide these when he appears.
Her (on-air — they'd seen each other before the show) reunion with him after the latter finally returned home from being kidnapped in Syria, as the two are extremely close in real life. She remains remarkably calm, but welcomes him home with genuine sincerity and then tells him, in typical Rachel fashion, that he is to remain safely at home and become a dentist now as getting himself kidnapped is just not on.
Whenever Rachel interviews him, she almost always opens or closes with, "Thank you for being with us," and Richard always replies "My pleasure" or "Absolutely" — but the intimacy with which he says it translates not to "this was a pleasant experience," but to "I absolutely love spending time with you and it's been so good to see you and I wish we could do this every day." It's a genuine pleasure for him to speak with her — and the warmth in her voice whenever she speaks to him proves that the affection is far from one-sided. And when he's somewhere dangerous, she usually says, "stay safe, my friend." Professionalism or no, these two adore each other.
How much fun they had together in Afghanistan and Iraq. Richard plainly adored having her there and getting to show her around, and Rachel was clearly exhilarated to see what he had been doing all those years and where he'd been living and what his life was like.
When he's showing her around Baghdad and telling her about the madness around the 2003 invasion, he briefly remembers a bomb that went off very close to his reporting position — then turns back to look at her with such a speaking glance that words are utterly superfluous. She sees the grief in his eyes as he remembers the danger and the colleagues and friends he'd lost, and simply pats him on the shoulder in a wordless gesture of support.
Rachel's smile on-camera is usually some blend of 'polite' and 'genuine', the exact ratio varying depending on who she's talking to — unless that person is Richard Engel, in which case 'polite' isn't even invited over for tea and she just lights up like a sunbeam until he's left again. It's mutual. And adorable.
Richard mock-indignantly squawking "I'm here for you, Rachel!" when she very seriously asked him if he could stick around through the commercial break and into the next segment, reducing Rachel to helpless laughter. In the immediate aftermath of the November 2015 terrorist attacks in Paris, that moment of warmth and levity was a badly needed bright spot.
Her entire relationship with Richard Engel, full stop. They positively light up whenever they're around each other (in person or by video link), their respect for each other is clearly enormous, and the sheer warmth emanating from them whenever they speak to each other/are in each other's presence could heat a house in Antarctica. Thank God for the Internet: someone made them a fanvid.
Richard Engel doesn't have a monopoly on adorable friendships with Rachel (even if theirs is the most magical). Her friendship with Chris Hayes is a thing of beauty and a joy forever.
Her tribute to the late Jeanne Manford, the founder of PFLAG. Plus the fact she admits that crying isn't a bad thing.
Doubles as a Tear Jerker, is her interview with the parents of one of the slain children at Sandy Hook Elementary. Watch here and here.
On a related note, following the release of the 911 calls of the day of the shooting, she swore that the show would not play any of the recordings, citing that they had nothing to contribute that could not be obtained in transcripts or interviews of those who had listened to them, and she does this on air, in a voice of smoldering anger at the fact that they have been released.
This extends beyond just 911 calls. Rachel hates the news media's tendency toward Bile Fascination, and so she absolutely refuses to join her colleagues in it. This includes her strict ban on any video or audio clips from the hostage executions carried out by ISIL during the Syrian Civil War, even when everyone else in the media (including her fellow MSNBC commentators) has done so.
Her August 6th, 2015 episode: was a farewell retrospective for Jon Stewart whose praises she sings.
20 January 2016: Nicolle Wallace, longtime Republican strategist (albeit one quite friendly with Rachel), goes out of her way to say that Rachel, who jumped on the Flint, Michigan water scandal early and didn't let up on it even when it was being ignored by the rest of the media, deserved a lot of credit for bringing national attention (and high-level government attention) to the crisis. Note that this had nothing to do with what Wallace was actually on the show to discuss.