Are 平常 and 通常 essentially interchangeable?
The two words share the same character &quot;常“(common, usual,eternal), so the meanings overlap somewhat. Both of them can mean &quot;commonly, usually&quot;, however, they are not exactly the same.
&quot;平常“ means (notice 平 means &quot;plain，commonplace&quot; here）

[adj]ordinary, casual/informal
[adv]ordinarily,in a casual/informal way

&quot;通常&quot; means

[adj]common,usual
[adv]in general, generally, in most cases

When something is ordinary, usually you could say it's the most possible case or it's common, that's how the two words have the similar meaning.
Examples:

我平常坐公交车去上班。 Usually I go to work by bus ( in my ordinary life).
我通常坐公交车去上班。 Usually I go to work by bus ( in most cases).
通常情况下，伤口的血液会自动凝固。 Generally, the blood from the wound coagulates automatically.
平常情况下，伤口的血液会自动凝固。 Sounds weird to me.
真理通常掌握在少数人手中。 The truth are held in few people in general.
真理平常掌握在少数人手中。 Sounds weird to me.
我平常叫他老王，但是在公司里我通常叫他王经理。 I call him Lao Wang ordinarily(casually), but I would usually call him Manager Wang in the company(because this is a serious situation).  Not interchangeable in this case.
我只想回到平常的生活。 I just want to return to a common\normal(in a style that average people have) life. Here, 通常 is not acceptable.