Charm Person has verbal and somatic components. Is there anything in the rules as written that defines how obvious those verbal and somatic components are? Would it be obvious the caster was casting a spell from 5 feet away? From across a crowded room?
I'm specifically interested in an answer based on the rules as written, although interpretation and individual opinion are not unwelcome.
In Chapter 10: Spellcasting of the rules, the Targets section states that spells with subtle effects can be cast without being noticed (PHB, p. 204; emphasis added):
Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature's thoughts, typically goes unnoticed, unless the spell says otherwise.
The casting of charm person is subtle enough that even the target doesn't know about the spell until it expires (PHB, p. 221):
When the spell ends, the creature knows it was charmed by you.
If even the target of the spell is unlikely to perceive that it was cast, other observers would have even less of a chance. (A court magician, trained in Arcana and watching for such things, would be a different case.)
Spells that are obvious when cast call that out in the description. For example, knock makes a loud noise that alerts nearby creatures.
Compare the description of burning hands:
As you hold your hands with thumbs touching and fingers spring, a thin sheet of flames shoots forth
...to that of dominate person
You attempt to beguile a humanoid [...]
These descriptions describe what casual onlookers will notice. You can try to charm or beguile someone without magic, it just might not be as effective.
Similar spells provide more detail. The material component of friends is described as:
A small amount of makeup applied to the face as the spell is cast
And after the friends spell expires, the target "realizes you used magic to influence its mood" - calling out the effect of the spell, not the components of the casting, as having become obvious.
Lastly, let's consider suggestion:
You suggest a course of action (limited to a sentence or two) and magically influence a creature
In this case the RAW are pretty clear; the physical manifestation of the spell is just that - there's no imp that appears on the target's shoulder to whisper in its ear.
Thus, when the wizard suggests to the head guard, "These aren't the constructs you are looking for," would we really expect the other guards to recognize the "combination of sounds, with specific pitch and resonance" in the old man's voice, and open fire?
School of Enchantment wizards gain the ability to make targets of their charms unaware of the spell after it was cast, but there's no mention of the spell-casting itself going from obvious to subtle, which one might assume would be part of the enchanter benefits if it were needed.
Also, if casting spells were tremendously obvious, many illusion spells would be not so useful.
Sorcerers have a Metamagic option called Subtle Spell, which allows them to spend sorcery points to cast the spell without verbal or somatic components.
This has the power to make otherwise obvious spells subtle. For example, a manacled and gagged spell caster suddenly transforms into a wolf-man and bursts his bonds. The guards think he's a werewolf, but he just used Subtle Spell and cast the alter self and knock spells.
The description of verbal components in the rules says:
Most spells require the chanting of mystic words.
And the description of somatic components says:
Spellcasting gestures might include a forceful gesticulation or an intricate set of gestures. If a spell requires a somatic component, the caster must have free use of at least one hand to perform these gestures.
Contrast this with the sorcerer's Metamagic option Subtle Spell, which allows a sorcerer to cast a spell without verbal or somatic components. We can reason that if removing these components makes a spell "subtle", then a spell without those components is not subtle.
Charm person has a casting time of one action, so it involves around five seconds of non-subtle chanting and forceful and/or intricate gestures with one hand. That sounds like something that would be noticed by someone standing next to the caster.
Generally speaking, I would say that casting a spell with verbal and somatic components is obvious if the caster is being observed, or to anyone standing close to them. For example, it's usually obvious when a spellcaster is casting a spell in combat. But in a crowded, noisy room, people not actively observing the caster might not notice a simple spell being cast.
Seeing that a spell is being cast isn't the same as knowing what the spell is. The rules don't specify how to handle this, but I generally allow an Intelligence (Arcana) check to identify a spell that a character observes being cast, provided they could reasonably know what the spell is. For spells that a character would have no knowledge of, I might allow them some information (e.g. "some kind of powerful invocation") on a successful check.
Charm Person has verbal and somatic components. Verbal components are defined as a "particular combination of sounds, with specific pitch and resonance, (which) sets the threads of magic in motion." Somatic components "might include a forceful gesticulation or an intricate set of gestures", and specifically requires the free use of at least one hand.
Neither of these things seem subtle to my reading. From 5' away they would be fairly obvious, although from across a crowded room they might be hidden. In the latter case the people next to the caster would likely observe these actions.
So the actions are obvious, but the meaning may not be. If bards are rare in the game world and people don't correlate fancy lutes with charms and spells, then perhaps the components will be treated as mere affectations. But if bards and wizards aren't merely the stuff of legend, even the simple peasants will have heard enough stories about clever bards and powerful wizards to understand that those strange gestures and odd songs are the stuff of magic. Those stories surely include some mention of the effects - few folks in the standard setting will be surprised that a spellcaster can charm people into strange behaviors. They will not recognize the specific spell as its cast, but people who witnessed the casting will be on guard, looking for possible effects.
Player's Handbook, p. 203, describes the various types of spell components. Of interest to us are:
Verbal... Most spells require the chanting of mystic words. The words themselves aren't the source of the spell's power; rather, the particular combination of sounds, specific pitch and resonance, sets the threads of magic in motion.
Somatic... Spellcasting gestures might include a forceful gesticulation or an intricate set of gestures
Both chanting and gesticulating forcefully are sound like they would be obvious to someone who was nearby. Across a crowded room they may be less obvious and perhaps a perception check would be appropriate.
If someone does want to perform magic both the Stealth and Sleight of Hand Skills appear to apply. Slight of Hand lists legerdemain as one of its uses which fits perfectly with hiding somatic components. Stealth makes mention of sneaking without being heard so perhaps fits well with masking chanting. Personally I call for a check for each component of the spell (Verbal, Somatic, and Material) since it nicely makes simpler spells easier to hide.
