- If your target succeeds on the Ref save, the sphere doesn't form in the first place: "Reflex negates", thus the effect (1-foot-diameter/level sphere centered around a creature) cannot be triggered, thus the sphere cannot form.
- Probably, and probably. The spell description is a bit vague, but it seems entirely reasonable that others could be trapped within the sphere (the description suggests this is the case: "The sphere cannot be moved ... by the struggles of those within.") and that those others would get a ref save to dive out of the way.
I advise conferring with the GM about how they would adjudicate such a sphere catching multiple creatures before spending too many resources on it (which may require casting the spell in a mock combat situation with your party).
If you're the GM, decide how it works and be consistent. It's fine if other spells (or other force-effects, even Forcecage) work differently, but make sure that Resilient Sphere always works the same (barring specific situational modifiers, of course).
Were I GMing, I would give non-targets who would be caught a ref save to dive out of the way, in the direction of their choosing (using up movement from their next action); an acrobatics check (or saving by 5+/hitting a nat-20 on the check) would let them end the movement standing, else they'd be prone. Large (or larger) creatures who are partially in the space would get the ref save to choose whether to end up inside or outside of the sphere; failure would put them on whichever side the caster wants them to be on.
GM ruling covers this: line of sight may be obscured
The rules don't specify whether the flaming sphere opaque or not, but there are some rules that can help the DM make a ruling on this situation (italic emphasis mine):
Making an Attack (SRD p. 94):
- Choose a target. […]
- Determine modifiers. The GM determines whether the target has cover and whether you have advantage or disadvantage against the target. In addition, spells, special abilities, and other effects can apply penalties or bonuses to your attack roll.
The GM makes a ruling on cover (yes/no) and how much cover(half, 3/4, total). What is available to assist in making this ruling? Cover and Vision rules offer some help.
Cover (p. 96 SRD):
A target with half cover has a +2 bonus to AC and Dexterity saving
throws. A target has half cover if an obstacle blocks at least half of
its body. The obstacle might be a low wall, a large piece of
furniture, a narrow tree trunk, or a creature, whether that creature
is an enemy or a friend.
A target with three‑quarters cover has a +5 bonus to AC and
Dexterity saving throws. A target has three‑quarters cover if about
three‑quarters of it is covered by an obstacle. The obstacle
might be a portcullis, an arrow slit, or a thick tree trunk.
A target with total cover can’t be targeted directly by an attack or a
spell, although some spells can reach such a target by including it
in an area of effect. A target has total cover if it is completely
concealed by an obstacle.
But the fire isn't solid, is it? (Neither is a portcullis, but I digress). You could shoot through the fire if you see some of the target behind there. But is there a penalty to the shot, since you can't see the whole target?
Look at the situation and decide if the flaming sphere provides some, a lot, total, or no cover.
- How much of the target can the Ranger see?
- How big is the target as compared to the flaming sphere?
- Does the Ranger have some room to move to get a better look before
releasing the arrow?
It will depend on the situation. In a 5' wide hallway, you could make the case for 1/2 cover, while in a large room where a Ranger can step to either side there may be no cover at all.
Why "cover" from a non-solid object?
Obscured vision. Is the target partly obscured by this big old ball of fire?
Vision and Light (SRD p. 86):
In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on
sight.
Foliage isn't opaque, per se, but it can certainly obstruct vision. You could consider the flaming sphere something like a burning bush: if not opaque, certainly an obstruction to vision.
Combine "obscured" with "cover" and you can make a good case for 1/2 cover in a situation where the Ranger can't move to get a clearer look at the target. I'd have a harder time arguing 3/4 cover or total, since the flame isn't a solid object.
Make a ruling, and play on!
Best Answer
Up to the DM
The spell says:
That's it. The rules don't say whether opaque or transparent. So the DM decides.
You can try to tie it in to other rules and build a logical structure that determines what the answer is, but the rules really just aren't that consistent.
So the DM decides.
Arguments for opacity
The spell says:
It's reasonable to assume that that means light and sound, they're energy, right? Basic physics. There you are in the sphere, and the only light or sound is from inside.
Counter-argument: "Physics"? Really? We broke physics back at the light cantrip. Earlier. Probably on page one of the Basic Rules.
Arguments for transparency
The spell says:
What's the point of being able to move it if you can't see where you're going?
Also, and it's a thin thread, I admit, part of the spell component says "clear crystal". Why say "clear" if the resulting sphere is opaque?
Counter-argument: Physics. It says energy can't go through. Light and sound are energy.
How to decide
Pick whichever feels most reasonable and go with it. You're not going to break the game either way. If you decide you don't like how it works, you can change your mind later.
Personally, I prefer transparent, because I think it seems more fun, and that's the way we've used it in games in the past.
In a game I currently play in, I have a wizard who has resilient sphere in their book, although I've never cast it. I've always assumed it would be transparent, because that's the way I've played it in the past, but I asked the DM. I got that DM look, and they said, "What? It doesn't say? Hmm. It's transparent, it's a hamster ball." So there you have it.
But it's your game, you decide.
As a player
If there's any doubt how it's going to work at your table, I recommend you ask your DM ahead of time, so that you can figure it out before you need to use it in session.